Re: [PATCH v2 3/4] fuse: add per-file DAX flag

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Jul 16, 2021 at 06:47:52PM +0800, Jeffle Xu wrote:
> Add one flag for fuse_attr.flags indicating if DAX shall be enabled for
> this file.
> 
> When the per-file DAX flag changes for an *opened* file, the state of
> the file won't be updated until this file is closed and reopened later.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>

[..]
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index 36ed092227fa..90c9df10d37a 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -184,6 +184,9 @@
>   *
>   *  7.34
>   *  - add FUSE_SYNCFS
> + *
> + *  7.35
> + *  - add FUSE_ATTR_DAX
>   */
>  
>  #ifndef _LINUX_FUSE_H
> @@ -449,8 +452,10 @@ struct fuse_file_lock {
>   * fuse_attr flags
>   *
>   * FUSE_ATTR_SUBMOUNT: Object is a submount root
> + * FUSE_ATTR_DAX: Enable DAX for this file in per-file DAX mode
>   */
>  #define FUSE_ATTR_SUBMOUNT      (1 << 0)
> +#define FUSE_ATTR_DAX		(1 << 1)

Generic fuse changes (addition of FUSE_ATTR_DAX) should probably in
a separate patch. 

I am not clear on one thing. If we are planning to rely on persistent
inode attr (FS_XFLAG_DAX as per Documentation/filesystems/dax.rst), then
why fuse server needs to communicate the state of that attr using a 
flag? Can client directly query it?  I am not sure where at these
attrs stored and if fuse protocol currently supports it.

What about flag STATX_ATTR_DAX. We probably should report that too
in stat if we are using dax on the inode?

Vivek